This invention relates to electric circuit breakers of the narrow width molded case type used in residential and commercial applications.
This type of circuit breaker generally has a handle for manually operating internal contacts between open and closed circuit conditions. In addition to the manual control, the circuit breaker is customarily provided with overcurrent responsive means which sense the current in the circuit being protected by the circuit breaker and operate to trip the circuit breaker to its contact open condition upon detection of currents exceeding a predetermined value. There are two types of overcurrent responsive elements generally employed in circuit breakers, the first being a heat sensing element which responds to low magnitude overcurrents of a prolonged duration. The second type is a magnetic trip device wherein overload currents of short duration and considerably higher magnitude electromagnetically energize a core to attract a magnetic armature thereto. Circuit breakers of this type generally employ both types of overcurrent responsive elements to afford a wide range of overcurrent protection.
The current levels available to residential and commercial customers is progressively increasing. The ability of a circuit breaker to interrupt increased current levels at high overcurrent or fault current conditions is significantly related to the separation speed of the contacts upon detecting such fault. The magnitude of currents now available to comsumers is such that separation of contacts under fault current conditions establishes a strong electrical arc between the contacts. It is necessary to quickly interrupt this arc to prevent harmful currents from being let through the circuit breaker.